Here are some threaded inserts or adapters between 1/4'' and M4, 3/8'' and M6 and 3/8'' and 1/4'' in heights of 5, 10 and 15 mm. And there is also a small key to get the inserts in and out again.
Make sure to have pretty well calibrated printer and filament. Also consider printing a brim.
The author marked this model as their own original creation.